Relationship between myosin heavy chain IId isoform and fibre types in soleus muscle of the rat after hindlimb suspension.

The relationship between the myosin heavy chain (HC) IId isoform and histochemically defined fibre types was investigated in the rat soleus muscle after hindlimb suspension. After 4 weeks of suspension, right and left muscles were removed and fibre type composition and total fibre number were examined by histochemical myosin adenosine triphosphatase staining sections. Myosin HC isoforms were analysed by sodium dodecyl sulphate polyacrylamide gel electrophoresis. After the suspension, there was a significant decrease in the percentage of type I fibres and a concomitant increase in that of type IIa fibres. However, the total number of fibres was not affected by suspension. The synthesis of HC IId isoform, which was not found in the control, and the decrease in the ratio of slow type myosin heavy chain isoform (HC I) were observed after suspension. These results would may suggest that the change of fibre type composition was caused by a shift from type I to IIa fibres after suspension. Furthermore, it could be suggested that the synthesis of HC IId isoform occurred during the stage of type shift from type I to IIa fibres.